In order for early cars and late cars to be treated the same, early cars have to have a more prominent splitter because they have a moreprominent bumpers. The current rule is based on a rule change request I made a couple years ago. The reason this is necessary is because a splitter and the front bumper work against each other and size matters.
At speed a high pressure zone develops at the front of the car. That high pressure zone pushes up against the underside of the bumper and causes lift. A splitter gives that same high pressure zone something to press down against, so the force pushing up is offset by force pushing down. That is to say, front lift is reduced.
Since a big early bumper traps a larger high pressure zone under it pushing it up, a larger splitter is required to provide a counteracting down force.
This isn’t the only role of a splitter, but it is the primary one.
The rule used to measure splitters from the body contour, but that prevented early model owners from installing a splitter proportional to their larger bumper.